約 and ぐらい: approximation before and after a quantity
Compare the placement of 約百人 and 五キロぐらい, keeping a number together with its counter or unit.
Place 約 before the quantity
約, read やく, precedes a quantity to mark approximation. 会場には約百人が集まった。 reports that about one hundred people gathered at the venue.
In 約百人, 約 modifies the full quantity 百人. 人 is the counter for people, so read it together with the number 百.
Attach ぐらい after the quantity
この箱は五キロぐらいあります。 says that this box weighs about five kilograms. ぐらい follows the combined quantity and unit 五キロ.
Learn 五キロぐらい as a unit rather than inserting ぐらい before キロ. The sentence concerns the box’s weight, not a count of five boxes.
Connect quantities to nouns and predicates
約百人の参加者 refers to about one hundred participants. 百人ぐらいの参加者 also connects an approximate quantity to a noun, with ぐらい after the quantity.
約五キロあります。 and 五キロぐらいあります。 place the approximation differently. Keep the number and unit together while distinguishing preceding 約 from following ぐらい.
Where does ぐらい go when expressing 約五キロあります。 with ぐらい?
Show the answer and why
After the combined number and unit 五キロ: 五キロぐらいあります。